Electronic components, such as semiconductor devices, are frequently tested, sometimes multiple times during their manufacture, using automatic test equipment. To perform these tests, automated test equipment may include instruments that generate or measure test signals such that a range of operating conditions can be tested on a particular device. An instrument, for example, may generate or measure a pattern of digital signals to enable testing of digital logic within a semiconductor device.
Modern semiconductor devices may have large numbers of test points, possibly hundreds or even thousands. Accordingly, the test system may have multiple channels, each designed to independently generate or measure a signal for a test point. The specific value to be generated or measured at each test point may be dictated by a test program that controls all of the channels of the test system. In addition to specifying the value of a test signal, the test program may specify a time at which the test signal is to be applied or measured. Being able to control the timing of test signals is important for thorough testing of a semiconductor device because a device that produces an expected value, but at the wrong time, can nonetheless cause a system using that semiconductor device to malfunction.
The time at which a test signal is to be applied or measured at one test point may be specified in relation to the time at which a test signal is applied or measured at another test point. Accordingly, it is frequently necessary that the test signals in multiple channels be coordinated. Test systems are designed to synchronize the generation of signals within different channels. Though, merely synchronizing the times at which signals are generated may not be adequate to ensure the test signals are coordinated at the test points of the device under test. Differences in propagation delay can change the relative timing of signals, thereby affecting the accuracy of test results. To increase testing accuracy, it is known to calibrate a test system. Calibration may entail measuring relative propagation delays through channels of the test system. The propagation delay through the channels can then be adjusted. Such calibration may be done at various times, including when a test system is manufactured or, once a test system is installed, on a periodic schedule or at times depending on an amount of use.